It's my impression that most "confectioner's sugar"/powdered sugar sold in supermarkets is 10X (ground 10 times) and that 4x & 6x are usually marked as such and found primarily in baking supply stores.

The bigger issue for me is that the label say "pure cane sugar". If it doesn't say that, it's usually beet sugar. I, personally, haven't been happy with the results of icings made with beet sugar. Domino's and C&H are marked "pure cane".

I read on the package of 10x (I am not sure the brand, but it came from Sam's Club) that it had cornstarch added to it to keep it from clumping. The regular ps did not,.

In the US, and I believe Canada as well, about 3% cornstrach is added to PS. In the UK, icing sugar doesn't routinely have cornstarch in it. Could be that with larger grains (4x & 6x), CS may not be necessary????